High pressure multistage centrifugal pump for fracturing hydrocarbon reserves

ABSTRACT

The present invention relates to a multistage centrifugal pump design, which has the diffusers, impellors, and a shaft, inserted within a high pressure housing, such that this assembly is fully enclosed within the housing, and the housing is of sufficient strength to be suitable for safe pressure containment of the fluids being pumped. This invention describes the technical details used to reconfigure the multistage centrifugal pump design to increase the discharge pressure capabilities higher than the 6,000 psig of current designs.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Ser. No. 13/353,353 entitled“High Pressure Multistage Centrifugal Pump for Fracturing HydrocarbonReserves” filed on Jan. 19, 2012, which is a continuation-in-part ofU.S. patent application Ser. No. 13/328,245 entitled “High PressureHydrocarbon Fracturing On Demand Method and Related Process” filed onDec. 16, 2011, which claims priority from U.S. Provisional ApplicationNo. 61/434,171 entitled “High Pressure Multistage Centrifugal Frac Pump”filed on Jan. 19, 2011, and U.S. Provisional Application No. 61/434,167entitled “High Pressure Multistage Centrifugal Pump for FracturingHydrocarbon Reserves” filed on Jan. 19, 2011, the entire disclosure ofwhich are hereby incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates in general to multistage centrifugal pumps forinjecting fluids into a wellbore, which has been drilled into reservoirrock formations, and in particular to multistage centrifugal pumpsinjecting fluids into wells for purposes of fracturing said wells. Inthe oil and gas industry, which utilizes fracturing operations tostimulate oil and gas reservoirs, this operation requires high surfacefluid treating pressures, which may be 10,000 psi.

BACKGROUND OF THE INVENTION

In oil and gas applications, fluids are frequently injected into awellbore for a variety of different purposes and various types ofsurface pumps are employed. In prior art, a multistage centrifugal pumpcould be mounted horizontally, at the surface, adjacent to, or nearbythe well requiring fluids to be injected into, and current designs havea maximum discharge pressure of 6,000 psi. This multistage centrifugalpump is one type of pump that is most often used in a verticalconfiguration within a wellbore for pumping fluid from the well tosurface pipeline systems, as a production pump, and current designs havea maximum discharge pressure of 6,000 psi. In the oil and gas industry,which utilizes fracing operation to simulate Oil and Gas reservoirs,this operation requires high surface fluid treating pressures, which maybe 10,000 psi. The present invention, a high pressure multistagecentrifugal pump, has been designed to increase the operating dischargepressure from 6,000 psi to 10,000 psi to enable this pump to meet theabove described application. This high discharge pressure capabilitycould also be applied to other applications too.

The prior art multistage centrifugal pump is used in the ElectricSubmersible Pumping System (“ESPS”) industry or in its surfaceHorizontal Pumping System (“HPS”) application, which are limited todischarge pressure or differential pressure between internal andexternal pressure of the housing, to be below 6000 psi. The o-rings arecommonly used as a sealing element between an intake and a pump base aswell as between a discharge and a pump head. The diffusers contain thepressure generated in the pump stages and the pump housing is only usedas secondary pressure containment since its primary role is to hold pumpcomponents together. The pump housing is sealed with o-rings on a pumpbase and a pump head. Diffusers are not designed to withstand highdifferential pressure between the outside and inside of the diffuser.

U.S. Pat. No. 3,861,825 teaches a multistage pump and manufacturingmethod. It describes the split-casing style of centrifugal pump. Thepump speed is listed as approximately 12,500 rpm, with a dischargepressure that may be 2600 psi, with a suction pressure of 15 to 30 psi.They do reference previous patents, and then list some patents that havesimilarities.

The pump Nexen has disclosed herein is a housing type of centrifugalpump, operating at speeds of 30 to 90 hz, (1800 to 5400 rpm), withdischarge pressures that may be 10,000 psi, and with a suction pressurethat may be 15 to 600 psi. Any similarities would be with respect tocentrifugal pumps in general, and the fact that they are composed ofmultiple stages.

U.S. Pat. No. 5,232,342 teaches high pressure multi-stage centrifugalpumps. It describes the split-casing style of centrifugal pump. Thatinvention relates to means for preventing rotation of an interstagebushing or ring, as the main objective. There is no reference in thispatent as to the discharge pressure capabilities to go along with the“High Pressure” referenced in the heading.

The pump Nexen has disclosed herein is a housing type of centrifugalpump, which is designed for operating at speeds of 30 to 90 hz, (1800 to5400 rpm), with discharge pressures that may be 10,000 psi, and with asuction pressure that may be 15 to 600 psi.

The main difference here is we are using a housing type of centrifugalpump and are building it with many more stages than what has been donein the past. The pressure capability far exceeds current designstandards (6,000 psi maximum listed by other manufactures such as Reda,Centrilift, Woodgroup, Weatherford, Canadian Advanced Inc.). CanadianAdvanced ESP Inc. (“CAI”) states in their HPS brochure that the HPSDesign Capacities are maximally 4600 psi. CAI used special constructiontechniques to meet Nexen design and specification requirements toaccommodate the high discharge pressure capabilities of 10,000 psiheretofore unknown.

With these ends in mind the main objective of the present invention isto provide details on pump construction that was used to expand themultistage housing centrifugal pump to enable it to operate at a veryhigh discharge pressure of 10,000 psi. The high pressure is contained bythe housing, which the diffusers are inserted into. High pressure iscontrolled through the use of seals on the external of the diffusers toprevent cross flow to other diffusers. Openings in external wall ofdiffusers are used to provide rapid release of pressure trapped betweenthe diffusers and the housing to prevent diffuser collapse when a unitis shut-down and depressurized. One skilled in the art would appreciatethe modifications provided in the present invention to achieve itsobjectives i.e. sufficient pressure control and pressure relief for thediffuser as required. This pressure release could be accomplished byslots, holes and other openings.

Special threading on the discharge ends of housings is required tosupport high pressure pipe connections.

It is yet another object of the invention to provide a multiple stagecentrifugal pump for fracturing hydrocarbon deposits that is capable ofgenerating in excess of 10,000 psi.

It is a further object of the invention to provide said pump designed toequalize pressures in the housing of said pump from stage to stage.

It is another object of the invention to provide said multiple stagecentrifugal fracturing pump with construction materials in alignmentwith the well known recommendations published for material performancecriteria from for example, NACE (National Association of CorrosionEngineers), ASTME (American Society of Tool and Manufacturing Engineers)or ANSI (American National Standards Institute) trim packaging or thelike in view of the corrosive nature of the fluids being pumped.

It is another object of the invention to provide said pump with thepreferred NACE trim packaging or the like in view of the corrosivenature of the fluids being pumped.

It is yet another object of the invention to provide a multiple stagehigh pressure centrifugal pump capable of use in fracturing ahydrocarbon reserve while avoiding treating the aquifer water prior tousing it for hydrocarbon fracturing as a result of the high pressurecapabilities of said pump.

It is a further object of the invention to enable the use of non-potableunderground aquifer water, such as the Debolt formation aquifer, as asource of water for the fracturing of underground rock formationscontaining hydrocarbon reserves.

Further and other objects of the invention will be apparent to oneskilled in the art when considering the following summary of theinvention and the more detailed description of the preferred embodimentsdescribed and illustrated herein along with the appended claims.

SUMMARY OF THE INVENTION

In this invention, a multistage centrifugal pump is built to be capableto deliver discharge pressure or differential pressure between pumpinternal and external pressures of up to substantially 10,000 psi ormore. A pump housing is designed to be the primary pressure containment.The sealing interface between the pump base and pump head is a metal onmetal type achieved by using specialized thread. The diffusers aredesigned with openings to allow rapid pressure equalization across thediffuser outside edge to avoid failure from high differential pressurewhich could cause diffuser failure. A seal is used on the outside of thediffusers to prevent pressure communication, and fluid flow, between theoutside of the individual diffusers enclosed within the housing. Thepump connections to pump intake and discharge are upgraded to ring orgasket style sealing.

The present invention also relates to a multistage centrifugal pumpdesign, which has the diffusers, impellers, and a shaft, inserted withina high pressure housing wherein this assembly is fully enclosed withinthe housing, and the housing is of sufficient strength to be suitablefor safe pressure containment of the fluids being pumped. This inventiondescribes the technical details used to reconfigure a known multistagecentrifugal pump design to enable increase of the discharge pressurecapabilities higher than the 6,000 psi of current designs. The designmodifications discussed herein have been successfully tested at 10,000psi discharge pressure. The 10,000 psi pressure capability provides apressure suitable for fracturing hydrocarbon formations penetrated bywellbores.

This style of pump unit is well suited to the hydrocarbon fracturingindustry to pump fluids at sufficient pressures, to stimulateunderground rock formations containing hydrocarbon reserves.

The invention is preferably a housing type of centrifugal pump, which isdesigned for operating at speeds of 30 to 90 hz, (1800 to 5400 rpm),with discharge pressures that may be 10,000 psi, and with a suctionpressure that may be 15 to 600 psi.

Preferably said pump includes a pressure sleeve (21) on top of diffuser(14) wall for improved wall strength by compression fit between sleeve(21) and outside diameter of diffuser (14) wall (FIGS. 3 and 4).

Also preferably said pump utilizes an equalization hole (23) in thediffuser wall, resulting in zero differential pressure across diffuserwall and also allows for rapid depressurizing (FIGS. 3 and 4).

Preferably to prevent stages from collapsing due to pressure transferfrom one pump stage to another, o-ring (31) style sealing is utilizedbetween each diffuser (14) and housing (16) (FIG. 3).

In one embodiment sealing between pump housing (16) and both pump base(12) and pump head (19) is by specialized threads providing metal onmetal sealing, eliminating all elastomeric and non-elastomeric sealsthrough the use of proven metal-to metal thread sealing technology suchas base-head pin-housing connection (FIG. 2).

The multistage centrifugal pump is designed for injecting fluids into awellbore for purpose of fracturing this well.

According to a primary aspect of the invention there is provided amultiple stage centrifugal pump for fracturing hydrocarbon depositscapable to deliver discharge pressure or differential pressure betweenthe pump internal and external pressure to be in the range of greaterthan 6,000 psi to up to substantially 10,000 psi or over, said pumpcomprising;

a pump housing designed for primary pressure containment,

a seal between the pump base and pump head being metal on metal typeachieved by using specialized thread,

diffusers designed with openings to allow rapid pressure equalizationacross the diffuser outside edge to avoid failure from high differentialpressure which could cause diffuser failure,

a seal used on the outside of the diffusers to prevent pressurecommunication, and fluid flow, between the outside of the individualdiffusers enclosed within the housing and the pump connections to thepump intake and discharge including upgrades to ring or gasket stylesealing. wherein said pump design delivers a discharge pressure ordifferential pressure between the pump internal and external pressure inthe range of greater than 6,000 psi to 10,000 psi or higher, which issubstantially much higher pressures then the previously 6,000 psimaximum limit.

Preferably the multistage centrifugal pump further comprises diffusers,impellers, and a shaft, inserted within a high pressure housing, theassembly being fully enclosed within the housing, and the housing beingof sufficient strength to be suitable for safe pressure containment ofthe fluids being pumped.

Another embodiment utilizes a pressure sleeve (21) on top of thediffuser (14) wall for improved wall strength by compression fit betweenthe sleeve (21) and the outside diameter of the diffuser (14) wall(FIGS. 3 and 4).

Yet another embodiment utilizes equalizations openings (23) in thediffuser wall, resulting in zero differential pressure across thediffuser wall which also allows for rapid depressurizing (FIG. 2).

Preferably to prevent stages from collapsing due to pressure transferfrom one pump stage to another o-ring (31) style sealing is utilizedbetween each diffuser (14) and housing (16) (FIG. 3).

More preferably the sealing between the pump housing (16) and both thepump base (12) and the pump head (19) is by specialized threadsproviding metal on metal sealing, thereby eliminating all elastomericand non-elastomeric seals through the use of proven metal-to metalthread sealing technology (base-head pin-housing connection see FIG. 2).

According to yet another aspect of the invention there is provided theuse of the pump described herein and above for a multi-stage centrifugalpump to provide mechanical and hydraulic pressure capability for thisHigh Pressure multistage centrifugal pump to operate at a range ofgreater than 6,000 psi to up to substantially 10,000 psi or moredischarge pressures for injecting fluids to a wellbore for the purposeof hydraulic fracturing of wells in hydrocarbon deposits.

The Debolt subsurface formation or zone in north east British Columbiais an aquifer whose water contains approximately 22,000 ppm of totaldissolved solids (“TDS”) and a small amount of hydrogen sulphide—H₂S.The scope and volume of the Debolt formation is still beinginvestigated, but it has the potential to be extensive. This aquifer hashigh permeability and porosity. A Debolt well at b-H18-I/94-O-8 wastested in May, 2010, with a 10.25″ 900 HP downhole electricalsubmersible pump (“ESP”). The well showed a Productivity Index of 107m3/d per 1 kPa drawdown, indicating that the reservoir will provide ahigh enough rate of flow to support the volume and rate requirementsneeded to support well fracturing operations.

Debolt formation water contains sour gas in solution. When depressurizedto atmospheric conditions, the Debolt water flashed off sour gas at agas water ratio of 1.35 standard m³ of gas to 1 m³ of water. The flashedgas contained 0.5% H₂S (hydrogen sulphide), 42% CO₂ (carbon dioxide) and57% CH₄ (methane). These gases are the same gases present in shale gasproduction wells, which are normally in the range of 0.0005% H₂S, 9%CO₂, and 91% CH₄, and the use of raw Debolt water would have anegligible impact on the current percentage of shale gas components.

The challenge is how to use sour water, for example Debolt water, forfracing in a cost effective manner since current water fracturingequipment does not comply with the well known recommendations publishedfor material performance criteria from for example NACE, ASTME or ANSIstandards for sour trim packaging or the like.

There are two different ways of using Debolt formation water forfracturing operations. The first is to construct and operate a watertreatment plant to remove the H2 S from Debolt water. This approach hasbeen taken by other industry participants who have constructed an H2 Sstripping plant to remove the H₂S from Debolt water. A recent paperpublished by Canadian Society for Unconventional Resources entitled“Horn River Frac Water: Past, Present, Future” discusses the technicaland operational aspects of the Debolt Water Treatment Plant constructedand operated for the foregoing purposes. This paper states that a veryexpensive treatment plant is required to remove the H₂S and othersolution gases from the Debolt water.

The second approach is to maintain the aquifer water at a pressure aboveits saturation pressure (also known as the “Bubble Point Pressure” or“BPP”) on a continuous basis while being produced to surface andtransported in pipelines to enable it to be used for fracturing. Testsconducted on the Debolt water properties indicates that as long as theDebolt water is maintained at a pressure high enough to keep thesolution gas entrained in the water, the water is stable with noprecipitates, and remains crystal clear in colour. Further, as long asthe Debolt water is kept above its BPP, then the water is in the leastcorrosive state. These findings reveal that the Debolt aquifer fluid canbe used in its natural state requiring no treatment. This is the basisof the proprietary Pressurized-Frac-on-Demand (“PFOD”) process.

A primary aspect of this invention is therefore to provide a method orprocess of fracturing a hydrocarbon deposit on demand comprising thesteps of:

using as a source of water an underground aquifer which contains waterwhich is stable and clear in the aquifer but which may includeundesirable constituents that are in solution when subjected to surfaceconditions such as hydrogen sulfide and other constituents,

utilizing the water from the aquifer as a source of water to be used ina hydrocarbon fracturing process and to pump the water under pressure ata predetermined rate for the aquifer water and above the bubble pointpressure (BPP) for the water contained in a particular aquifer to keepthe water stable. We have found that the water becomes unstable when thepressure is reduced and gas is allowed to evolve out of the water. Thisdepressuring and gas removal initiates a chemical reaction with thedissolved solids in the water to cause precipitates to form. To preventthese chemical reactions from occurring and causing the undesirableconstituents of said water from falling out of solution,

maintaining said water pressure at a minimum required for each aquiferat all times during the fracturing process,

drilling a source well into the aquifer,

drilling a disposal well to the aquifer,

providing a pump capable of maintaining the required pressure needed toprevent the constituents of the aquifer water from coming out ofsolution only by maintaining the minimum pressure,

establishing a closed loop with a manifold, or a manifold and pumps, tokeep the aquifer water circulating at all times until the fracturingoperation begins when water will be supplied from that manifold,

providing the fracturing operation with water from the manifold so as tofracture a hydrocarbon reserve,

wherein in using water from an aquifer in the fracturing process and bymaintaining said water under pressure at a minimum at all times, saidwater remains stable and the undesirable constituents remain in solutionand the water remains clear thereby avoiding the necessity of treatingthe water from the aquifer prior to using it in a fracturing processes.

According to another aspect of the invention there is provided a methodor process of high-pressure fracturing of a hydrocarbon deposit, forexample a shale gas deposit on demand comprising the steps of using as asource of water from an underground aquifer such as the Debolt aquiferwhich contains sour water including H2 S and other constituents,

utilizing the sour water from the aquifer as the water source to be usedpreferably on at least the clean side of a gas fracturing process and topump said sour water under pressure at a minimum of for example 2310 kPafor Debolt water at approximately 38 degrees Celsius (which varies withthe actual temperature of source water for each aquifer, and any surfacecooling which may occur to such water) and above the BPP for the sourwater contained in a particular aquifer to prevent H₂S and otherconstituents of said sour water from falling out of solution,

maintaining said sour water pressure at a minimum required for eachaquifer, for example for Debolt of 2310 kPa at all times during thefracturing process,

drilling a source well into the aquifer,

drilling a disposal well into the aquifer,

providing a pump capable of maintaining the required pressure needed toprevent the constituents of the sour water from coming out of solutiononly by maintaining the minimum pressure required which, for example,for Debolt water is 2310 kPa at 38 degrees Celsius,

establishing a closed loop with a manifold to keep the sour watercirculating at all times until the well fracturing operation begins whenwater will be supplied from that manifold, or a manifold and pumps,

providing the clean side of a well fracturing operation with sour waterfrom the manifold so as to fracture a well reserve (normally an oil orgas zone reserve),

wherein in using sour water from an aquifer such as Debolt for the wellfracturing process and maintaining said sour water under pressure at aminimum, as an example for Debolt water being at 2310 kPa and 38 degreesCelsius, said water remains stable and the constituents remain insolution and the water remains clear thereby avoiding the necessity ofstripping out the hydrogen sulfide and other constituents as is requiredby other well fracturing processes.

In one embodiment of the invention said water source and method orprocess is utilized along with sand on the dirty side of the wellfracturing operation with the addition of a high-pressure blender sincethe sour water must be maintained above its BPP, for example 2310 kPafor Debolt water at 38 degrees Celsius at all times thereby avoiding theconstituents including the H2 S from falling out of solution.

In a further embodiment of the method or process the necessary number ofpumps and source wells and disposal water wells are provided with themethod or process to enable a high-pressure fracturing operation ondemand for a target number of fracs (which depends on the particularwell design chosen for a reservoir stimulation or other purpose) foreach well, or number of wells, stimulated as part of a program.

Preferably in the method or the process said water from the sourceaquifer is at an elevated temperature (as compared to surface watertemperatures), for example for Debolt water a temperature under normalcircumstances has been 38 degrees Celsius, which therefore requires noadditional heating, or insulated piping, and which may be used as asource of sour water for the pressurized fracturing on demand processeven during the colder winter months experienced in, for example,Western Canada or similar areas and which can contribute considerablecost savings when compared to utilizing surface water.

In yet another embodiment the method or process utilizes sour water fromthe Debolt aquifer and continuously circulates said water at a pressureabove the BPP from the source well to the disposal well in anunderground pipeline system accomplished by a back pressure controlvalve located downstream of the well to be fractured near the Deboltwater circulation line and yet upstream of the disposal wells whereinwhen water is required for frac operations, water will be withdrawn froma manifold strategically located on this circulation line therebyfeeding Debolt water to the frac operation under pressure, which isabove the Debolt BPP.

According to yet another embodiment of the method or process the Deboltwater is maintained at a pressure above its saturation pressure (“BPP”)and is continuously used for fracing so that as long as the Debolt wateris maintained at a high enough pressure to keep the solution gasentrained in the water, then the water remains stable, with noprecipitates and is in the least corrosive state thus requiring that allfrac operations (at least on the clean side) be conducted at pressuresabove the Debolt water BPP which is the basis for a successful PFODprocess.

In yet another embodiment the method or process further comprises a NACEtrim, preferably a High Pressure Horizontal Pumping System (“HPHPS”)frac pump capable of providing a discharge pressure of about 69 MPa. Thepump construction uses materials in alignment with the recommendationspublished by the National Association of Corrosion Engineers (“NACE”)trim packaging in view of the corrosive nature of the fluids beingpumped). Alternatively, materials may be selected from materialperformance criteria for a HPHPS frac pump or equivalent published byfor example ASTME, ANSI or the like. Alternatively, other suitable pumpconstruction materials can be tested specifically for the fluid topumped to ensure suitable material compatibility is maintained.

In order to carry out the process of this invention, a multistagecentrifugal pump is built capable of delivering a discharge pressure ordifferential pressure between pump internal and external pressures toover 10,000 psi. A pump housing is designed to be the primary pressurecontainment. The sealing interface between the pump base and pump headis a metal on metal type achieved by using specialized thread. Thediffusers are designed with openings to allow rapid pressureequalization across the diffuser outside edge to avoid failure from highdifferential pressure which could cause diffuser failure. A seal is usedon the outside of the diffusers to prevent pressure communication, andfluid flow, between the outside of the individual diffusers enclosedwithin the housing. The pump connections to pump intake and dischargeare upgraded to ring or gasket style sealing.

The present invention also relates to a multistage centrifugal pumpdesign, which has the diffusers, impellers, and a shaft, inserted withina high pressure housing, wherein this assembly is fully enclosed withinthe housing, and the housing is of sufficient strength to be suitablefor safe pressure containment of the fluids being pumped. This aspect ofthe invention describes the technical details used to reconfigure theknown multistage centrifugal pump design to enable increase of thedischarge pressure capabilities higher than the 6,000 psi of currentdesigns. The design modifications discussed herein have beensuccessfully tested at 10,000 psi discharge pressure. The 10,000 psipressure capability provides a pressure suitable for fracturingformations penetrated by wellbores.

This style of pump unit is well suited to the hydrocarbon fracturingindustry to be used to pump fluids at sufficient pressures, to stimulateoil and gas reservoirs.

The invention is a housing type of centrifugal pump, which is designedfor operating at speeds of 30 to 90 hz, (1800 to 5400 rpm), withdischarge pressures that may be 10,000 psi, and with a suction pressurethat may be 15 to 600 psi.

Preferably said pump is utilizing pressure sleeve (21) on top ofdiffuser (14) wall for improved wall strength by compression fit betweensleeve (21) and outside diameter of diffuser (14) wall (FIGS. 3 and 4).

Also preferably said pump is utilizing equalizations hole (23) indiffuser wall, resulting in zero differential pressure across diffuserwall and also allows for rapid depressurizing (FIG. 2).

Preferably to prevent stages from collapsing due to pressure transferfrom one pump stage to another o-ring (31) style sealing is utilizedbetween each diffuser (14) and housing (16) (FIG. 3).

In one embodiment sealing between pump housing (16) and both pump base(12) and pump head (19) is by specialized threads providing metal onmetal sealing, eliminating all elastomeric and non-elastomeric sealsthrough the use of proven metal to metal thread sealing technology suchas base-head pin-housing connection (FIG. 2).

The multistage centrifugal pump is designed for injecting fluids into awellbore for purpose of fracturing this well.

According to that aspect of the invention there is provided a multiplestage centrifugal pump for fracturing hydrocarbon deposits capable todeliver discharge pressure or differential pressure between the pumpinternal and external pressure to be over 10,000 psi and including apump housing designed for the primary pressure containment, sealingbetween the pump base and pump head is metal on metal type achieved byusing specialized thread, diffusers are included designed with openingsto allow rapid pressure equalization across the diffuser outside edge toavoid failure from high differential pressure which could cause diffuserfailure, a seal is used on the outside of the diffusers to preventpressure communication, and fluid flow, between the outside of theindividual diffusers enclosed within the housing and the pumpconnections to pump intake and discharge are upgraded to ring or gasketstyle sealing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view (an isometric view) of high pressuremultistage centrifugal pump unit constructed in accordance with thepresent invention.

FIG. 2 is a sectional view of high pressure multistage centrifugal pumpassembly illustrating components used within assembly.

FIG. 3 is a sectional view of a portion of high pressure multistagecentrifugal pump embodying the present invention.

FIG. 4 is a sectional view of a diffuser for the high pressuremultistage centrifugal pump embodying the present invention.

FIG. 5 is a PFOD Process Flow Schematic.

FIG. 6 is a PFOD Elevation View of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Over the past two years, Nexen has been working on the PFOD process asoutlined below, using Debolt water above its BPP for fracing thuseliminating the need for an expensive H₂₅ removal process.

In order to guarantee a reliable source of water for its fracturingoperations, it was necessary to identify ways to utilize the Deboltwater as part of the frac water source. One of the options reviewed wasto use Debolt water for only the clean side of the frac program.

In light of its requirements, Nexen designed and built a small flow highpressure multistage centrifugal pump for testing. In June 2010, a 0.25m³/min NACE trim high pressure multistage centrifugal test pump capableof providing a discharge pressure of 69 MPa was tested on the b-18-I padin northeast British Columbia. Technicians were onsite to operate theDebolt water source well (“WSW”) ESP and the high pressure multistagecentrifugal test pump. Three chokes consisting of two bean types and onevariable choke were piped up in series to provide the back pressure totest the high pressure multistage centrifugal pump at fracturingpressure.

In the initial tests, the high pressure multistage centrifugal test pumpused freshwater from a tank truck. All the pump control parameters wereset. In subsequent tests, Debolt water was used and fed by the DeboltWSW at b-H18-I/94-O-8 by ESP to the suction of the high pressuremultistage centrifugal test pump. The discharge from the test pumpflowed through three chokes at various back pressures. The Debolt waterthen exited the chokes and flowed into a disposal water pipeline to thewater disposal well (“WDW”) at b-16-I. The back pressure wasprogressively increased at 7000 kPa intervals and ran at that dischargepressure for approximately 30 to 60 minutes. When pump operationsremained steady, the choke was adjusted to increase the dischargepressure of the pump.

The high pressure multistage centrifugal test pump was successfullytested on Jul. 7 and 8, 2010. It operated at a maximum dischargepressure of 71 MPa. The pump was run using Debolt water forapproximately 6 hours at 62 MPa to simulate a complete fracturingoperation.

It is understood that other aquifers will have different physicalparameters. For example, pump specifications will reflect different BPPfor alternative water sources. For the Debolt water source, the BPP ofthe aquifer water was 2310 kPag at 38 degrees Celsius.

In August 2010 during the completion of the 8 wells at pad b-18-I, thehigh pressure multistage centrifugal test pump was integrated into 6fracturing operations. Three of the 6 fracs ran using freshwater and 3ran using Debolt water. The high pressure multistage centrifugal testpump ran well for all 6 fracs and there were no operational or safetyissues encountered.

Only one source water well and one disposal well are required for theinitial testing of the PFOD system, and additional wells will provideincreased capacity and backup to ensure minimum flow rate and injectioncapacities are available as required for the system to operate reliablywith maximum system availability and use. Nexen is planning to drill andcomplete additional Debolt formation WSWs and additional Debolt WDW inthe future as required to optimize the Debolt water system to supportfracturing operations. Together with the existing b-H18-I Debolt WSW andthe existing Debolt WDW b-16-I, these 2 initial wells plus anyadditional wells will form the basis of the PFOD water circulationsystem identified for such well fracturing program.

Nexen will continue to further evaluate the need to source and test a1.25 m³/min full size 3000 kPa suction pressure for a sour trim plungerfrac pump for the dirty side based on the well known recommendationspublished for material performance criteria from for example, NACE,ASTME or ANSI trim packaging or the like. This also includes theevaluation of the need for a pressurized blender, or another method forutilizing Debolt water for the dirty side.

Based on the Debolt water well tests conducted in June 2010, afeasibility study of the PFOD process, and initial field testing of aprototype NACE sour trim high pressure multistage centrifugal frac pumpin July and August of 2010, it was concluded:

-   -   It is technically and economically feasible to use Debolt water        in its untreated state for fracturing operations.    -   It is possible using the PFOD process to maintain pressures        above 2310 kPa (BPP for Debolt water) thus keeping gases        including H2 S contained in solution.    -   No water compatibility issues have arisen using Debolt water for        fracturing or injection into underground hydrocarbon shale        reservoirs.    -   A high pressure multistage centrifugal sour trim frac pump using        Debolt water can be constructed and used on the clean side of        fracturing operations.    -   No operational or safety issues were identified during the        testing and ultimate use in the field of the high pressure        multistage centrifugal pump.    -   Freshwater may not be readily available for operations. Water        from Debolt using PFOD process is readily available, and        availability is not subject to spring and summer rainfall or        suspension of licenses due to drought. For example, in August,        2010, government regulators in British Columbia suspended        freshwater withdrawal licenses for hydrocarbon fracturing        operations in the Montney area due to a drought in the Peace        River watershed.    -   There is experience in the pump industry in building a high        suction pressure plunger style pump, with a NACE sour trim fluid        end. There is no experience in the frac pump industry in        building a high suction pressure (over 330 psi (2300 kpag))        plunger style frac pump, with a NACE trim fluid end, capable of        pumping American Petroleum Institute (“API”) quality frac sand        for the dirty side fracing.    -   There is no apparent technical limitation or constraint to        prevent the engineering and fabrication of a pressure blender to        use Debolt water under pressure.

The PFOD Process Illustrated in FIGS. 5 and 6.

The PFOD process maintains water at a pressure above its BPP at alltimes in order to prevent gases (including H₂S, C02 and CH4) from comingout of solution. Based on Debolt well formation water andPressure-Volume-Temperature (“PVT”) tests, the Debolt water BPP is 2310kPa (335 Psi) at 38 degrees Celsius. When the Debolt water at 38 degreesCelsius was depressurized to atmospheric pressure, approximately 1.35 m₃gas was released per m₃ of water.

The flashed gas contained 0.5% H₂S, 42% CO₂ and 57% CH₄ (methane). Theseare the same gases present in certain shale gas operations (normally0.0005% H₂S, 9% CO₂, and 91% CH₄ (methane). The use of raw Debolt waterwould have negligible impact on the current percentage of shale gascomponents content.

For the typical PFOD system, 1 or more Debolt WSWs and 1 or more DeboltWDWs will be required. Debolt water will be continuously circulated at apressure above the BPP from the WSWs to the WDWs utilizing a pressurizedpipeline system. This will be accomplished by a back pressure controlvalve located downstream of the well to be fractured and near the waterdisposal well wherein when water is required for frac operations, waterwill be withdrawn from a manifold strategically located on thiscirculation line thereby feeding Debolt water to the frac operationunder pressure, which is above the Debolt BPP. The two figures show aPFOD flow schematic and a subsurface elevation view. These figuresdemonstrate how the PFOD pipeline system would work.

The advantages of a PFOD process are numerous and include the following:

-   -   Fracturing operations can to be conducted on a continuous basis        year round. Debolt water is typically at 38 degrees Celsius.        This allows for the use of Debolt water in the winter months        without requirement for heating or the other infrastructure        often required for winter frac operations including insulated        pipelines for water circulation.    -   Year round fracing capability will allow for production        flexibility relative to commodity demand and pricing.    -   The PFOD process eliminates the intensive capital and operation        costs associated with building, operating and maintaining water        treatment facilities.    -   The PFOD process also reduces the need for secondary facilities        that are required as development of fracturing operations occurs        at greater distances from the water treatment and H₂S removal        plants.    -   The PFOD process eliminates the need for above ground treated        water storage tanks or large holding ponds that would ordinarily        be required to heat the water for an above ground treatment        process. The Debolt aquifer therefore acts as a natural storage        tank with no surface facilities, heating or maintenance        required.    -   The Debolt aquifer could also be used as the main storage        location of excess fresh water to be used later during a        fracturing operation.

Referring to the drawings and in particular to FIG. 1 shown therein is apreferred embodiment of the high pressure multistage centrifugal of thepresent invention. Depending upon the design pressure required theassembly is composed of one or more multistage centrifugal pumps (45) ofthe preferred high pressure multistage centrifugal pump (46). Pumpbrackets (10) attaches pumps (45) & (46) to a base (9) which serves as afoundation for complete assembly. A motor (42) is attached to the pumps(45) through a thrust chamber assembly (43). The assembly (20) also hasintake (44) & discharge (47) which are suitably rated pressurecomponents that allow the pump assembly to mechanically connect toexternal piping while directing and controlling flow within said piping.

FIG. 1 illustrates a schematic view of the high pressure multistagecentrifugal pump assembly describing and numbering all components usedwithin the assembly including:

-   9 the pump support—skid frame-   42 the pump driver—electric motor-   43 the thrust chamber to support shaft load from pump-   44 the pump intake section-   45 the low pressure multistage centrifugal pump housings containing    the diffusers, impellers and shaft. Two pump sections are shown.-   46 the high pressure multistage centrifugal pump housing containing    the diffusers, impellers and shaft. This is an inventive aspect that    takes the pressure capability from 6,000 psi up to up to    substantially 10,000 psi discharge pressure.-   47 high pressure discharge head for 10,000 psi. This is another    inventive aspect that takes the pressure capability from 6,000 psi    to up to substantially 10,000 psi discharge pressure.

The high pressure multistage centrifugal pump (46) is an assembly ofimpellers (13) and diffusers (14). The impellers (13) are installed onpump shaft (15) and are rotating as part of the shaft, as the impellersare mechanically connected to the shaft. The diffusers (14) are fixed inthe pump assembly by being compressed by compression bearing (18) in thepump housing (16) against the pump base (12.) In order to increase thepressure produced to 10,000 psi discharge pressure, a sufficient numberof impellor and diffuser stages are stacked on each other to increasethe head capability of one stage to create the pressure required of allstages combined.

FIG. 2 is a cross-section of the high pressure multistage centrifugalpump assembly of FIG. 1 describing all components used within theassembly including pump base (12) and pump head (19) threaded into pumphousing (16). Each pump stage is an assembly of impeller (13) anddiffuser (14). The impellers (13) are installed on pump shaft (15) andare the rotating part of the pump. The diffusers (14) are fixed in thepump assembly by being compressed by compression bearing (18) in thepump housing (16) and against pump base (12).

The sealing between pump housing (16) and both pump base (12) and pumphead (19) is achieved by specialized threads such as API (AmericanPetroleum Institute) or Hydril threads providing metal to metal sealcapabilities under high differential pressure environments. High torquemake up ensures strong connection capable of taking axial hydraulic loadfree of the leak. Each connection is also designed to withstand multiplemake-ups and breaks without requiring redress.

Attention is next directed to FIG. 3 which shows a preferred embodimentof the invention. The high pressure multistage centrifugal pump (46)includes an outer high pressure housing (16) that holds and aligns allthe components of the pump. The high pressure multistage centrifugalpump (46) includes diffusers (14) which are constructed with supportsleeve (21) completely around the diffuser, which has grooves (25) ando-ring (31) the housing (16), and thereby provides a seal within saidhousing. When the pump is operating there is always some leakage intothe annulus formed by the inside diameter of housing and outsidediameter of the diffuser (14). When the annulus becomes full flow intoit ceases as the pressure in the annulus equals the pressure at thesource of the leak. If the source of the leak is at or near thedischarge head of the pump, the annulus can be pressurized to fulldischarge pressure. To prevent this condition o-rings (31) are installedat every diffuser and equalization holes (23) are placed throughdiffuser wall so that maximum pressure is not limited by thin wallthickness of diffusers.

FIG. 3 is a cross-section illustration of FIG. 2 showing a number ofimpeller and diffuser stages in the high pressure multistage centrifugalpump housing (16). This invention includes the equalization hole (23)for rapid depressurizing, and the support sleeve (21) completely aroundthe diffuser, which has grooves (25) to contain the o-ring (31) toprevent pressure communication, and fluid flow, between the outside ofthe individual diffusers enclosed within the housing. This high pressurehousing (16) is designed to safely contain pressures up to 10,000 psi.

FIG. 4 illustrates in cross section the details of each diffuser (14),the support sleeve (21), the equalization hole (23), and the o-ring (31)for the high pressure multistage centrifugal pump assembly and thediffuser details showing compression sleeve (21) on top of diffuser(14). This invention includes the equalization hole (23) for rapiddepressurizing, and the o-ring (31) to prevent pressure communication,and fluid flow, between the outside of the individual diffusers enclosedwithin the housing.

The present invention offers an economy of manufacture while affordingmaximum serviceability at the site of installation throughout the use ofa high pressure multistage centrifugal pump. A presently preferredembodiment has been described for purposes of this disclosure.

The multistage high pressure centrifugal pump is to be built in such waythat it eliminates high pressures across diffusers (14) wall by theprovision of equalization openings (23) and sealing each diffuser in thehousing, and improving diffuser wall strength (FIG. 2) wherein thepressure is contained by the pump housing (16) (FIG. 3).

The generic pump will contain pump base (12) and pump head (19) threadedinto pump housing (16). A pump stage is an assembly of impeller (13) anddiffuser (14). The impellers (13) are installed on pump shaft (15) andare the rotating part of the pump. The diffusers (14) are fixed in thepump assembly by being compressed by compression bearing (18) in thepump housing (16) and against pump base (12) (FIG. 2).

There are two options for improving the diffuser (14) wall strength:

1. utilizing increased wall thickness (improved wall strength) and tight(few thousandths of an inch) fit between the diffuser and the housing,thus preventing diffuser deformation.

2. As shown in FIG. 3 utilizing pressure sleeve (21) on top of diffuserwall (14) (improved wall strength by compression fit between sleeve andoutside diameter of diffuser wall) and tight (few thousandths of aninch) fit between diffusers (14) and housing (16), thus preventingdiffuser deformation.

Elimination of pressure gradient across the diffuser wall is achieved bydrilling equalizations hole (23) in the diffuser wall resulting in zerodifferential pressure across diffuser wall (14). To eliminate a higherpressure from one stage to act on other diffusers, o-ring (31) stylesealing is utilized between each diffuser (14) and housing (16),preventing pressure transfer, or fluid flow, on top of the diffusers(14) from one end of the pump housing to another. The primary pressurecontainment is the pump housing (16) (FIG. 3).

The sealing between pump housing (16) and both pump base (12) and pumphead (19) is achieved by specialized threads such as API or Hydrilthreads providing metal on metal sealing, utilizing a large torqueshoulder to permit high torque make-up to ensure strong connection,maximize material cross section resisting burst. The connection isdesigned to withstand multiple make-ups and breaks without requiringredress.

Sealing between piping and the pump discharge is by using ring or gaskettype sealing and API type flanges (11) (FIG. 2).

The multistage centrifugal pump can be built as a single pump (low TDH)or as a multi-section pump (high TDH) (FIG. 4), depending on requiredTotal Dynamic Head (TDH). In the multi-section design, pumps sections(45, 46) are connected in series on common pump bed (9) and their shaftsare mechanically connected to be driven by common driver (42). Thethrust generated in the pump is contained by Thrust Bearing Assembly(43). Pump intake (44) and discharge (47) complete the assembly.

The design modifications discussed herein have been successfully testedat a 10,000 psi discharge pressure. The 10,000 psi pressure capabilityprovides a pressure suitable for fracturing formations penetrated bywellbores.

As many changes therefore may be made to the preferred embodiment of theinvention without departing from the scope thereof. It is consideredthat all matter contained herein be considered illustrative of theinvention and not in a limiting sense.

What is claimed is:
 1. A method for hydraulically fracturing an underground hydrocarbon deposit comprising: using an underground aquifer as a source of water for a hydrocarbon fracturing process, the water containing undesirable chemical compounds as soluble components that are not in solution when subjected to pressure at atmospheric conditions; pumping the water under pressure at a predetermined level for the aquifer water and above a bubble point pressure for the water contained in the aquifer to prevent the undesirable chemical compounds in the water from separating out of solution; maintaining the water pressure above the bubble point pressure during a fracturing operation; drilling a source well into the aquifer; drilling a disposal well into the aquifer; pumping the water within a closed loop to maintain the water above the bubble point pressure; circulating the water until a fracturing operation begins; and conducting the fracturing operation with water from the underground aquifer to fracture the hydrocarbon deposit, wherein the water is maintained above its bubble point pressure, said water remains stable and the undesirable chemical compounds remain in solution.
 2. The method of claim 1, wherein: the water remains clear during said conducting step.
 3. The method of claim 1 wherein: fracturing is conducted for a shale gas deposit.
 4. The method of claim 1, wherein: the water includes hydrogen sulfide, and pumping of the water under pressure and above the bubble point pressure prevents the hydrogen sulfide from separating out of solution.
 5. The method of claim 1, further comprising: establishing a closed loop circulation during said circulating step by use of a manifold, or a manifold and at least one pump to keep the water circulating until conducting the fracturing.
 6. The method of claim 1 further including; providing at least one source well and at least one disposal water well enabling fracturing to occur on demand for each pad of a predetermined number of pads.
 7. The method of claim 1 wherein: the water from the aquifer is kept at an elevated temperature in relation to ambient surface water.
 8. The method of claim 5 further comprising: providing an underground pipeline including a water circulation line and a back pressure control valve located in the water circulation line, wherein the pipeline communicates with the manifold or the manifold and at least one pump, such that water can be withdrawn from the manifold or the manifold and at least one pump.
 9. The method of claim 1 further comprising: providing a high pressure blender to maintain the water above its bubble point pressure.
 10. A method for hydraulically fracturing an underground hydrocarbon deposit comprising: using an underground aquifer as a source of water for a hydrocarbon fracturing process, the water containing undesirable chemical compounds as soluble components that are not in solution when subjected to pressure at atmospheric conditions; pumping the water under pressure at a predetermined level for the aquifer water and above a bubble point pressure for the water contained in the aquifer to prevent the undesirable chemical compounds in the water from separating out of solution; maintaining the water pressure above the bubble point pressure during a fracturing operation by pumping the water with a multiple stage centrifugal pump, the pump including (i) a pump base, (ii) a pump head, (iii) a metal on metal housing seal located between the pump base, and pump head, and (iv) a plurality of diffusers having openings to allow rapid pressure equalization across a diffuser outside edge to avoid failure from high differential pressure, each diffuser having a seal located on an outside portion of each of the diffusers to prevent pressure communication and fluid flow between the outsides of the individual diffusers enclosed within the housing; circulating the water until a fracturing operation begins; and conducting the fracturing operation with water from the underground aquifer to fracture the hydrocarbon deposit, wherein the water is maintained above its bubble point pressure, said water remains stable and the undesirable chemical compounds remain in solution.
 11. A method of claim 10, further comprising: drilling a source well into the aquifer; drilling a disposal well into the aquifer; and pumping the water within a closed loop to maintain the water above the bubble point pressure.
 12. A method of claim 10, wherein: pump connections of said pump for pump intake and discharge include ring or gasket sealing, and wherein said pump delivers a discharge pressure or differential pressure between the pump internal and external pressure of up to substantially 10,000 psi or over.
 13. A method of maintaining adequate pressure in hydraulic fracturing of an underground hydrocarbon deposit comprising: maintaining water pressure for water in an underground aquifer used in a fracturing operation above a bubble point pressure for the water during the fracturing operation by pumping the water with a multiple stage centrifugal pump, the pump including (i) a pump base (ii) a pump head (iii) a housing including a housing seal located between the pump base and pump head (iv) a plurality of rotating impellors and (v) a plurality of stationary diffusers each having openings to allow rapid pressure equalization across a diffuser outside edge to avoid failure from high differential pressure, each diffuser further having a seal located to an outside portion of each of the diffusers to prevent pressure communication and fluid flow between the outsides of the individual diffusers enclosed within the housing.
 14. A method of claim 13, wherein: the housing seal includes a metal on metal type housing seal.
 15. A method of claim 13, wherein: pump connections of the pump used for pump intake and discharge include ring or gasket sealing, and wherein the pump delivers a discharge pressure or differential pressure between the pump internal and external pressure of up to 10,000 psi or over.
 16. A method, as claimed in claim 13, wherein: the pump further includes a pressure sleeve secured to an outside wall of each of said diffusers by a compression fit to prevent deformation of the diffusers during operation of the pump.
 17. A multistage high pressure centrifugal pump comprising: a pump base; a pump head; a housing including a housing seal located between the pump base and pump head; a pump shaft disposed in the housing; and a plurality of pump stages mounted along the pump shaft in series, each pump stage including a rotating impellor mounted on the pump shaft and a stationary diffuser having an opening to allow rapid pressure equalization across an outside edge of the diffuser to avoid failure from high differential pressure, wherein
 18. A multistage high pressure centrifugal pump according to claim 17, wherein: each diffuser is fixed in the pump by a compression bearing seated against the pump base.
 19. A multistage high pressure centrifugal pump according to claim 17, wherein: each diffuser further includes a diffuser seal located on an outside portion of the diffuser to prevent pressure communication and fluid flow between the outsides of the individual diffusers enclosed within the housing.
 20. A multistage high pressure centrifugal pump according to claim 17, wherein: the opening of the diffuser is an equalization hole extending radially outward from a longitudinal axis of the pump housing.
 21. A multistage high pressure centrifugal pump according to claim 17, wherein: pump connections connected to the pump and used for pump intake and discharge include ring or gasket sealing, and wherein the pump delivers a discharge pressure or differential pressure between the pump internal and external pressure of up to 10,000 psi or over.
 22. A multistage high pressure centrifugal pump according to claim 17, further including: a pressure sleeve secured to an outside wall of each of said diffusers by a compression fit to prevent deformation of the diffusers during operation of the pump.
 23. A multistage high pressure centrifugal pump according to claim 22, wherein: the diffuser seal includes a groove formed on the pressure sleeve for each stage, and an o-ring sated in the groove. 